Within the scope of the present invention, it is meant by vulcanized rubber, a raw elastomer (for example polyisoprene) which has been subjected to a chemical method during which it has been formulated with additives of which a vulcanizing agent (for example sulfur) allowing the vulcanization and having undergone a thermal treatment in such a manner as to create bridges between the macromolecules of said elastomer to form a three-dimensional network.
It results from the vulcanization a less plastic material but more elastic than the starting raw elastomer. Meaning that as a result of an appropriate constraint, the three-dimensional network becomes deformed. It resumes its initial state when the constraint is removed, and thus thanks to the presence of bridges formed during the vulcanization which can be modeled by springs.
The vulcanization is an irreversible transformation, achieved at temperatures generally ranging between 150 and 350° C., as beyond this value of 350° C., most elastomers would become damaged. The damage would translate by a change in the visco-elastic properties of the elastomer with, in general, an increase of the viscous component and a decrease of the elastic component.
After vulcanization, the vulcanized rubber is infusible. In fact, the vulcanization is the achieving of chemical bonds, generally sulfur-based, between the macromolecular chains of the elastomer. The origin of these chemical bonds is the rupture of a residual double bond of a macromolecule. It is worth noting that the vulcanized elastomer still has, within it, numerous residual double bonds, which are just as many sites which can be later used for new chemical reactions.
The fact that the vulcanized rubber is infusible later prevents any new shaping of an article manufactured in such a material, which may be required, for example, in the case of producing a non compliant article or for an article at the end of life or even for recycling production offcuts.
This is why, to this day, the only recycling operations commonly applied on articles manufactured from vulcanized rubber are grinding or shredding in order to obtain a powder, a crumb or an aggregate of elastic material. The elastic material thus obtained may then be used as elastic filler or diluted filler in various products. In no case can it be chemically bonded with a polymeric matrix.
Thus, in light of the limited industrial applications of ground vulcanized rubber and especially due to the fact that the shaping operation is impossible after the vulcanization, it can be easily understood that the recycling of articles manufactured from vulcanized rubber remains a technical and economic issue of major importance, and thus with regard to the permanent increase in production of such articles in vulcanized rubber.
Certainly, it is known to subject a very harsh chemical and thermal treatment during which the three-dimensional network of the vulcanized rubber is dismantled. However, the product obtained from such a treatment, called “regenerated rubber”, has less efficient physical and mechanical properties than the original vulcanized rubber. This limits its usage in production methods requiring materials which should comply with drastic requirements. This is why, this method of recycling vulcanized rubber is not fully satisfactory.
Furthermore, several scientific works have been conducted on methods of partial thermo-mechanical degradation of a vulcanized rubber, in other words called “partial devulcanizing methods”. In this respect, patents describe such methods which implement partial and monitored degradations of a vulcanized rubber by an important simultaneous shearing and a pressure increase.
By way of examples, it may be cited the international application WO 02/05965 A1 and the French patent application FR 2 947 555 A1 which, however state methods requiring a lot of energy.
Furthermore, the technologies introduced in these two patent applications are tricky to implement and exhibit a low reproducibility by virtue of the phenomena inherent to a thermo-mechanical treatment which initiates partial degradations of the elastomer.
In fact, it is known by the skilled person that the phenomena of deterioration to the macromolecular chains lead to creating free radicals of which the life expectancy is zero. This principle, known by the name Quasi-steady-state-approximation (abbreviated “QSSA”), specifies that the free radicals will disappear by recombining quasi instantaneously with other free radicals present in the immediate vicinity thereof. Hence, there is creation of local micro-domains of which the structure is random and non reproducible. Likewise, this reaction which is primed at high temperature, will continue according to a kinetic which is much lower at room temperature, leading to an instability in time of the properties of the thus devulcanized rubber. In other words, the product resulting from this thermo-mechanical treatment is chemically degraded and unstable and hence not fully satisfactory.
Furthermore, the international application WO 2012/017414 A1 describes a thermo-mechanical treatment of elementary particles of rubber in an extruder of co-rotary twin-screw type. The extruder used is equipped with long screws, of which the ratio of length of the screws on the diameter of the screws is higher than 64. The thermal treatment is carried out at one single temperature ranging between 35° C. and 350° C., preferably of the order of 270° C. The shape of the different elements of the screw allow to make the rate of shear vary and thus create locally the self-heating required for the rupture of the bridges resulting from the vulcanization.
However, the method described in this international application WO 2012/017414 A1 does not allow achieving an optimal devulcanizing. In fact, it exhibits the following two major drawbacks:                Owing to the typology of the applied treatment (cutting the vulcanization bridges by increasing the rate of shear in an enclosure maintained at constant temperature), it is not to be considered to have a homogenization in temperature of the material during the shearing operation allowing the devulcanization thereof.        Similarly, it is not possible to immediately lower the temperature of the material after devulcanizing and thus block the start of thermal degradation pertaining to the local presence of a temperature higher than the decomposition temperature of the rubber.        
The material thus obtained does not exhibit a homogenous chemical structure, but deteriorated micro-domains, rendering any later re-vulcanizing operation very uncertain.
It is also worth noting that the material which exits from the extruder is at a high temperature which is a function of the inner temperature of the extruder and the shearing to which it has been subjected. This material thus often exhibits the drawbacks of being sticky and self-adherent. If its temperature is very high (namely higher than 250° C. or 350° C. according to the rubber treated), it is liable to become deteriorated in an irreversible manner.
Moreover, it should be noted that the partial thermo-mechanical degradation methods such as that of the international application WO 2012/017414 A1 lead to the rupture of the majority of the bonds resulting from the vulcanization and the aforementioned residual double bonds, of which the main focus has been detailed above, thus rendering any later re-vulcanizing operation impossible.
Thus, it has been noticed from the prior art described above that none of the developed technologies has allowed to find a fully efficient, cost-effective and easy to implement solution for recycling articles in vulcanized rubber.